A steam generator is a plant for the generation of steam from a flow medium. In such a plant a flow medium, typically water, is heated and converted into steam. The steam is then used to drive machines or to generate electricity. Usually a steam generator comprises an evaporator to generate the steam and a superheater, in which the steam is heated to the temperature required for the user. Frequently a preheater is arranged upstream of the evaporator to make use of waste heat, and further increases the efficiency of the entire plant.
In industrial use today steam generators are usually designed as water-tube boilers, i.e. the flow medium is fed into steam generator pipes. The steam generator pipes can be welded together in a gas-tight fashion and thus form a surrounding wall, within which the hot gas supplying the heat is fed. Steam generators can be of either a vertical or horizontal construction, i.e. the hot gas is fed in a vertical or horizontal direction.
Steam generators can furthermore be designed as forced-flow steam generators, wherein the passage of the flow medium is forced by a feed pump. The flow medium is fed into the boiler by the feed pump and flows through the preheater, the evaporator and the superheater in succession. The heating of the feed water to saturated steam temperature, the evaporation and superheating take place continuously in a single flow, so that—at least when operating at full load—no distinct separation system for water and steam is necessary. Steam generators can also be operated at supercritical pressures. The definitions of the individual heating surfaces of preheater, evaporator and superheater are strictly speaking no longer appropriate in this operating mode, as a continuous phase transition takes place.
In a variant of the vertically piped through-flow steam generator the pipework of the surrounding wall is divided into a lower and an upper section, wherein the lower section comprises a first multiplicity of steam generator pipes in parallel configuration and the upper section a second multiplicity of steam generator pipes in parallel configuration, in series configuration with and downstream of the first multiplicity. The lower and the upper section are connected to each other by a passage collector. By this means on the one hand equalization of pressure between the steam generator pipes in parallel configuration is obtained, and on the other hand at least partial mixing of the flow medium from different steam generator pipes as well.
In the case of such through-flow steam generators with steam generator pipes and passage collectors traversable by flow in the vertical direction it has now been ascertained that individual pipes in the upper section of the surrounding wall can assume inadmissibly high temperatures, which under certain circumstances can result in a deterioration of the pipe wall, with the occurrence of these excessive temperatures being associated with certain operating parameters.